PROBLEM TO BE SOLVED: To infect an insect with a recombinant virus, into which a gene of a useful substance is transferred, in a high infection ratio in a short period, by orally inoculating a free virus and a spindle body of insect poxvirus into an insect.
SOLUTION: A spindle body of insect poxvirus is added to a solution of a free virus, preferably a nuclear-polyhedrosis virus of free silkworm, etc. An insect (silkworm, etc.) takes the solution by a method for adsorbing the solution on the feed of the insect, applying the solution to mulberry leaves, etc. In the case of preparing a recombinant virus derived from a nuclear-polyhedrosis virus, a useful gene is inserted into a gene part encoding a protein forming an inclusion body to give a recombinant virus in a free virus state.